The built-in or so-called internal volume of a rotary compressor should maintain a specific relation to the pressure ratio of the compressor if optimum efficiency is to be obtained. It must therefore be possible to vary the internal volume depending on whether full load or partial load prevails. The internal volume can be controlled, for instance, by an axially adjustable slide. Stepwise control of the internal volume can be performed, for instance, using several radially adjustable lift valves. Stepless control of the internal volume, however, usually requires an extremely complicated control system. Such a system for control of the internal volume may comprise a hydraulic system with solenoid valves and some form of calculating unit, for instance a processor, which controls the solenoid valves depending on the prevailing pressure ratio. The solenoid valves in the hydraulic system are then opened or closed to allow different pressure levels access to a hydraulic motor connected to an adjustable slide, for instance, to position this and thus control the internal volume.